E4561. Too Much of a Good Thing: Accessory Muscles of the Upper Extremity
  1. Jennifer Padwal; Stanford University Medical Center
  2. Kathryn Stevens; Stanford University Medical Center
Accessory muscles of the upper extremity are often overlooked, particularly on fat-suppressed MR images, but can have a significant clinical impact and present intraoperative challenges if not identified on preoperative imaging. Prevalence of various accessory muscles widely vary, with some muscles such as the extensor digitorum manus brevis present in 1 - 3% of patients compared to the accessory abductor digiti minimi muscle which is estimated to be present in nearly 50% of patients. Accessory muscles are best identified on MRI, particularly on T1 or PD-weighted images without fat suppression, where they can be confidently separated from the adjacent fat. Detailed knowledge of upper extremity muscle anatomy and awareness of these normal variants will help the practicing radiologist correctly identify these accessory muscles and their relationship to upper extremity pathology.

Educational Goals / Teaching Points
The goals of this educational exhibit are to review muscular variants in the upper extremity and discuss their clinical relevance.

Key Anatomic/Physiologic Issues and Imaging Findings/Techniques
MR images will be used to illustrate accessory muscles of the upper extremity including the accessory infraglenoid muscle in the shoulder, Gantzer muscle and anconeus epitrochlearis at the elbow, and anomalous palmaris longus, accessory abductor digiti minimi, extensor digitorum manus brevis, and accessory flexor carpi radialis brevis vel profundus at the forearm and wrist. These accessory muscles may be mistaken for soft tissue neoplasms and can also be associated with various pathologies including median or ulnar neuropathy or quadrilateral space syndrome.

Upper extremity accessory muscles are typically incidentally noted but can have important clinical implications. Identification of accessory muscles in the upper extremities requires a detailed knowledge of normal anatomy and can be missed if the radiologist is unaware of these normal variants. This exhibit will provide an MRI review of upper extremity accessory muscles and their clinical significance and serve as a helpful reference for the practicing radiologist.